10-Hydroxy-20-(S)-camptothecin is known to have a pharmacological activity itself (cf., for example, Non-patent Document 1) and also is serviceable as a starting material for synthesizing 7-ethyl-10-hydroxy-20-(S)-camptothecin which is an important intermediate in the synthesis of irinotecan. In the Non-patent Document 1, 10-hydroxy-20-(S)-camptothecin is prepared by a two-step procedure via photoreaction of camptothecin-1-oxide. The previously proposed process for preparation of 10-hydroxy-20-(S)-camptothecin involved oxidizing 1,2,6,7-tetrahydro-20-(S)-camptothecin, using an oxidizing agent selected from the group consisting of lead tetraacetate, CAN (cerium ammonium nitrate), Fremy's salt (potassium nitroso disulfonate (KSO3)2NO), chromic acid or anhydride, dichromate salts, potassium permanganate, ferric chloride and iodosobenzene diacetate (Patent Documents 1 and 2). Patent Document 1 discloses that 1,2,6,7-tetrahydro-20-(S)-camptothecin which is the starting material can be obtained by hydrogenation of 20-(S)-camptothecin in acetic acid or dioxane-acetic acid in the presence of a platinum catalyst at ambient pressure and temperature.
On the other hand, Patent Document 2 refers to the process of said Patent Document 1 and indicates the following: “Such a method is not fully satisfactory, however, due to the fact that the hydrogenated product is reactive. Hydrogenation therefore continues beyond the preparation of the desired tetrahydrocamptothecin product, resulting in the formation of over-reduction products. Additionally, while the use of a supra-atmospheric hydrogen pressure, although not disclosed in the Japanese patent publication, would be desirable from the standpoint of obtaining a more rapid reaction and/or enhanced conversion of the camptothecin starting material, it has been found that such pressures may not effectively be employed in a process such as that of the Japanese patent due to a further increase in the formation of over-reduction products”. With the view to improve the defect in the hydrogenation method of the Patent Document 1, Patent Document 2 proposes to use a noble metal catalyst in the presence of a hydrogenation catalyst moderator selected from hydrogenation catalyst poisons such as dimethylsulfoxide, or to use the noble metal catalyst in combination with a special carrier such as 5% platinum-on-sulfided carbon.
It is also known that 7-ethyl-1,2,6,7-tetrahydro-20-(S)-camptothecin can be obtained by contacting 7-ethyl-20-(S)-camptothecin with hydrogen and noble metal catalyst in the presence of a hydrogenation catalyst moderator selected from hydrogenation catalyst poisons such as dimethylsulfoxide (cf. Patent Document 3).
Results of studying the use of a large variety of heterogeneous catalysts in hydrogenation of quinolines, which can be regarded as a model reaction for selective hydrogenation of camptothecins having quinoline skeletal structure, have been reported (Non-patent Document 2). This document suggests necessity of concurrent use of a sulfur compound (e.g., CS2, H2S) or CO with a noble metal catalyst and nickel catalyst for selectively hydrogenating the heterocyclic ring (pyridine ring) only of quinolines, in conformity with the selective hydrogenation of camptothecins as described in Patent Documents 2 and 3.